Source: Kentucky Pest News
Author: Don Hershman

University of Kentucky Plant Disease Diagnostician, Paul Bachi, has just returned from a fact-finding and learning trip to Brazil to observe soybean rust first-hand. Paul returned with many insights and with a wealth of information regarding the Brazilian soybean rust situation. One observation that Paul made was that some of the things we are saying here about managing soybean rust with fungicides are not necessarily true for Brazil. For example, Paul learned that Brazilian soybean growers typically would not apply solo chlorothalonil or strobobilurin sprays. They do, however, frequently apply solo triazoles and premixes of a strobilurin plus a triazole. Paul’s observation is confirmed by some the literature he brought back published by EMBRAPA (more or less like our USDA) and from the Matto Grosso Foundation (an extremely large grower cooperative). Literature from both groups clearly indicates a significant bias towards triazoles and/or pre-mix products, and an aversion to spaying solo strobilurins and chlorothalonil.

If you attended any of our 11 Soybean Rust Intensive Trainings, or read anything that I and many other soybean pathologists have been saying about fungicide use for rust control, you know quite well that we have made room for possible use of both chlorothalonil and solo strobilurin fungicides. Based on the Brazilian experience, this appears to be a flawed concept.

As I have reflected on this apparent contradiction, I have come to understand what may be at its root. Brazil, being close to the equator, does not experience the dramatic seasonal temperature variation that occurs in most of the U.S. Because of this, much of Brazil is not subject to the hard killing frosts (28oF and below) which are so typical in the U.S. The net effect is that in Brazil, rust-infected volunteer soybean and other infected legume hosts are not killed. This means that newly-planted soybeans in Brazil are always likely to be near a source of soybean rust from the time of emergence to maturity. This is significantly different than the U.S situation where soybean rust will die back each year (like it did this winter) to the extreme southern portion of the country. Unlike Brazil, our soybean rust epidemics will require spores of the fungus to move substantial distances, especially when one considers the soybean crop from Kentucky and northwards. The main point is that soybeans will not be generally exposed to soybean rust spores throughout the season as happens in Brazil. Doublecrop soybeans may be an exception to this general truth.

Now that I have established a very large difference between the U.S. and Brazil, let us return to the apparent fungicide use contradiction. I believe that what the Brazilians are saying by their actions is that because their soybean crops are always near a source of soybean rust spores, they do not have any confidence that they can establish a truly “pre-infection” program. Thus, they have come to rely very heavily on the curative activity of triazioles, (solo and in mixes) and have learned through experience that by the time they see rust, the “pre-infection” strategy has already been defeated. We, on the other hand, should have significantly less difficulty establishing a truly pre-infection program since spores have to move into an area before an epidemic can be initiated. The net effect for us is that we may have better results with products (solo strobilurins and chlorothalonil) that are only effective pre-infection. Perhaps the greatest challenge facing U.S. soybean producers attempting to implement a pre-infection strategy is to not apply fungicides too early.

Time will tell if what I have said above is accurate. But at least for now, I believe we have valid reason to believe that pre-infection fungicide use strategies will work better here than they do in Brazil.